Production of alloy surface castings



Patented Mar. 21, 1933 UNITED STATES PATENT oar-"E JAMES H. GRITCHETT, OF DOUGLASTON, NEW YORK, ASSIGNOR TO ELECTRO METAL- LURGIGAL COMPANY, A CORPORATION OF WEST VIRGINIA PRODUCTION OF ALLOY SURFACE CASTINGS No Drawing.

The invention relates to the production of able high-chromium content.'

tion composed of steel or iron and alloy coatings which are high in chromium and are resistant to abrasion and corrosion. These alloy coatings may contain 12% or more of chromium but they commonly contain about 20% to of chromium. The castings are made by pouring the steel or iron into a mold which is covered with ferrochromium powder on the part or parts which correspond to the part or parts of the casting to be coated.

Heretofore alloy coated castings have been made by casting iron or steel into moldswhich were covered with a composition composed of powdered ferrochromium and a binder. Ferrochromium which contains about 70% chromium, 20% to 25% iron, 0.5% to 2% silicon, I carbon is easily reduced to powder ofthe desired sizes for this purpose. It alloys easily with the base metal and it has a suit- Consequently this high carbon ferrochromium alloy has universally been used for the purposes indicated. Frequently the alloy coatings made with this powder were defective, especially at or near the tops of surfaces which were disposed in a vertical position when the base metal was poured. Portions of the surfaces would be irregular or indented, presumably as a result of shrinkage during cooling and these portions, would be found to be uncoated with the high-chromium alloy. It then became necessary to fill in the uncoated portions by welding, using a filler rod which would deposit a composition similar to that of the coating.

The main object of the invention is to provide means for preventing the described discontinuity in high-chromium alloy coatings.

Experience 'has shown that the use of high-carbon ferrochromium for the coating of the mold requires that the temperatures and heat capacity of the casting as a whole be regulated within narrow limits. Whilev these conditions could be influenced to some extent by adjusting the pouring temperatures of the cast metal by the use of chills, the described defects could and about,,4% to 6%" Application filed MarchBO, 1931. Serial No. 526,551.

not always be overcome because it was not always possible to regulate the pouring temperatures or chilling in foundry practice within a sufficiently narrow range to give a heat balance which would produce perfect coatings. Even in castings where it was possible to regulate the heat balance by these means, it was costly to determine the exact pouring temperatures and the exact location for the chills. lVitha change in the size or shape of the casting the determination of new pouring temperatures or new chilling conditions was necessary.

The following example is illustrative of my invention. F errochromium containing about 67% to 70% chromium, 0.5% to 2.0% silicon and 1.0% to 1.25% carbon, and the balance chiefly iron waspowdered. Particles which passed through a number 20 sieve and were retained on a number 48 sieve were dampened with a sodium silicate solution and a layer of the composition about onesixteenth to one-eighth of an inch thick was placed on a mold which has a normally vertically disposed wall. The mold was dried and carbon steel containing about 0.5% carbon was poured into the mold. The cooled casting was found to have a continuous coating of high chromium alloy which could not be chipped from the base metal and no bare spots due to shrinkage were present.

It has been found that the defect can be overcome by using ferrochromium powder which contains about 1.0% to 2.72% carbon. Steel or iron can be used for the base metal, the invention being more advantageously applied in connection with base metals which contain relatively less carbon than the ferrochromium powder. The best results are generally obtained when the carbon content of the powder, the amount which the base metal dilutes the coating, and the carbon content of the base metal are regulated so that the carbon content of the surface alloy will be considerably below 2.7 say about 1.0% carbon. The greatest variations in pouring temperatures can be used when the carbon content of the coating alloy tends to be reduced continuously as the base metal continues to alloy with the ferrochromium powder, but the breaks in the coating can be avoided with wide variations in pouring temperatures even when the higher carbon base metals are used and the carbon-in the coating is built up approximately to 2.7% carbon.

- The exact reasons for the improvements which result from the use of the described low carbon ferrochromium are unknown to me. It is my present belief that the low carbon ferrochromium rapidly forms a coating alloy which solidifies more rapidly than the base metal when the temperature begins to drop and that a strong non-rupturing surface coating is formed before the base metal has had time to cool sufficiently to shrink. The formation of this non-rupturing surface alloy attemperatures above 'the shrinkage temperature of the base metal avoids the necessity for close regulation of the pouring temperatures and of other conditions which affect the heat balance.

The practicable pouring temperatures commonly range from about 2650 t0 3000 F. It is necessary to use a powder which is sized so that the speed of combination of the core metal with the powder at the pouring temperatures and the proportions of base metal and powder which combine will be sufficient to produce an alloy coating with the desired chromium content and with such a perfect joining together that the layer of alloy cannot easily be chipped off, sealed off or separated from the base metal on bending or breaking. The powder is sized to exclude the coarsest and the finest particles. The largest particles are preferably of about the size which will just pass through a number 20 sieve and the smallest are those of about the size which will just be retained on sieve numbers 50 to 80. Various selected sizes within these limits may be used, for example particles which will pass through a number 30 sieve and will be retained on a number sieve may be used. The sieve numbers in the specification and claims refag to the U. S. standard sieve numbers a opted by the American Society for Testing Materials.

It is to be understood that alloy steels and irons may be used for the base metal, that-the vertically disposed portions as well as the entire surface of the mold may be coated with the low carbon ferrochromium, that variations in the thickness of the coatings of powder and binder and in the grading of the size of the particles may be made to vary the thickness and character of the alloy coatings, and that the content of chromium and iron may be varied within wide limits without departing from the invention. The ferrochromium may range from about 60% to 97% chromium, the remainder except about 0.5% to 2.0% silicon and about 1.0% to 2.72% carbon being iron.

and claims refers to all the parts of the matrix which are used to give shape to the casting and may include cores as well as the parts of molds which constitute receptacles. Unless otherwise indicated, the term, lining, as used in the specification and claims refers to the coating of powder which is placed on the mold surface.

I claim 1. The method of producing a casting having base metal composed of ferrous alloy and a coating composed of a chromium-containing alloy formed by the union of the cast metal and a lining of ferrochromium powder disposed on the casting mold, which comprises lining the mold with ferrochromium powder containing about 1.0% to about 1.25%

carbon and pouring into the mold ferrous alloy which contains carbon, but not more than 0.5% carbon.

2. A coating for molds adapted to form a chromium-containing alloy surface on a ferrous alloy base metal cast in the mold, which comprises a ferrochromium powder containing carbon, but not more than 2.72% carbon.

3. A coating for molds adapted to form a chromium-containing alloy surface on a ferrous alloy base metal cast in the mold, which comprises a ferrochromium powder containing about 1.0% to about 1.25% carbon.

4. A coatin'g for molds adapted to form a chromium-containing alloy surface on a ferrous alloy base metal cast in the mold, which comprises a ferrochromium powder of such a size that the smallest particles will be retained on about number 40 to number 80 sieves and the largest particles will pass through number 16 to number 20. sieves, said powder containing carbon, but not more than about 2.72% carbon, about 0.5 to 2.0% silicon, about 60% to 97% chromium, and the remainder iron.

In testimony whereof, I aflix my si nature.

JAMES H. CRITCH- TT. 

